For this type of explosion-proof housing, it is permitted that an explosion can occur in the interior of the housing and the resulting explosion pressure in this instance, having a pressure of, for example, 40 bar, must not expand outwards into the atmosphere. For this purpose, such housings are flame-proof. For housings composed of a plurality of parts, the ignition spark resulting in the interior is not to expand out into the environment. For this purpose, the joining gaps between the interlocking housing parts are designed having fit dimensions with narrow tolerances. The joining gaps according to DIN EN 60079-1 (VDE 0170-5) must have such a length and (small) width so that an explosion pressure resulting in the interior of the housing extinguishes in the joining gap (subsequently also referred to as ex-joint) and cannot expand into the atmosphere.
Hence, it is ensured that a flash-over of the explosion occurring in the proofed interior does not carry-over into the atmosphere—which, if applicable, is enriched with a flammable mixture—surrounding the sensor.
The company Pepperl+Fuchs has introduced cylindrical rotary encoder housings as part of their 78-E series which can be used in an explosive environment. Such rotary encoder housings differentiate themselves in that two housing halves are connected with screws distributively disposed around the circumference, and the screws and their stud bolts are situated outside of the flame-proof housing.
Such a rotary encoder housing has an undesirably large outer diameter of approximately 100 mm. The large diameter results from the fact that the mounting screws are situated radially outside of the flame-proof, inner housing. For this reason, additional housing planes have to be created radially outside of the flame-proof area as joint planes having ex-joints which are penetrated by the mounting screws holding the housing planes together. With regard to a rotary encoder housing having an axial turning shaft running into the rotary encoder housing in the vertical direction, the known rotary encoder housing provides in the radial (horizontal) direction joining gaps (ex-joints) extending outward which enlarges the outer diameter in an undesirable manner.
Furthermore, other explosion-proof housings for rotary encoders and other sensors having more than two housing parts, which are in the vertical direction above one another and releasably connected to one another via joining gaps, are known, which are sealingly connected by mounting screws radially outward of the pressure-proof area. Such coupling areas extending radially outward between the releasably connected housing parts increase the overall diameter of the sensor housing in an undesirable manner.